1. Field of the Invention
The invention relates in general to a method of forming a polysilicon layer on a substrate, and more particularly to a method of forming a polysilicon layer with larger grain size on a substrate.
2. Description of the Related Art
On the substrate of thin film transistor liquid crystal display (TFT-LCD), there are a number of TFT Arrays, which function to control the arrangement of the liquid crystal of each pixel. The thin film transistor is mainly made of amorphous silicon or polysilicon. The polysilicon is superior to the amorphous silicon in its higher electron mobility. This favorable characteristic enables the polysilicon to integrate higher number of circuit. The complexity and weight of the product are therefore decreased. Thus, the polysilicon technology becomes a main stream in the field of TFT-LCD development and the task of increasing electron mobility of polysilicon is now a priority issue for researchers.
FIGS. 1A to 1D illustrate a conventional method of forming a polysilicon layer on a substrate. First, referring to FIG. 1A, a buffer layer such as a silicon dioxide layer 120 is first deposited on the substrate 110 by the technique of plasma enhanced chemical vapor deposition, PECVD. Next, referring to FIG. 1B, an amorphous silicon layer 130 is deposited on the silicon dioxide layer 120 by the same technique, PECVD. Amorphous silicon is precursor of polysilicon. Finally, referring to FIG. 1C, a laser beam 140 is emitted onto the amorphous silicon layer 130. Laser beam 140 serves as a thermal resource to heat and convert amorphous silicon 130 into polysilicon layer 150 as shown in FIG. 1D
Referring to FIG. 1C, The laser beam with a wavelength of 308 nm combined with ELA technique is used to crystallize the amorphous silicon layer on the substrate 110; Thus, the amorphous silicon layer 130 is transformed to the polysilicon layer 150 as shown in FIG. 1D. The buffer layer, such as the silicon dioxide layer 120, serves as a heat insulator between the substrate 110 and the amorphous silicon layer 130. With the benefit of the buffer layer, the temperature of the substrate 110 will not exceed 250° C. even though the temperature of the amorphous silicon layer 130 approaches 1500° C. during the ELA process. Moreover, the substrate 110 would not be deformed because the rapid heating process, which takes merely about several hundred microseconds.
The substrate mentioned above can be a glass substrate. The method of forming a polysilicon layer on a substrate can be applied to a TFT-LCD manufacturing process. The buffer layer is either a silicon dioxide (SiO2) layer or a silicon nitride (Si3N4) layer.
In the conventional method of forming a polysilicon layer on a substrate, the high speed of heat conduction attributes to the high thermal conductivity (TC) of the substrate 110 as well as the buffer layer. Accordingly, the thermal energy of the melted amorphous silicon (liquid silicon) dissipates rapidly through the substrate 110 and the silicon dioxide layer 120, which limits the duration of crystallization and consequentially influences the grain size of the polysilicon. The grain size of the polysilicon layer 150 directly ruins the performance of the thin film transistor, such as lower electron mobility.
In conclusion, the electron mobility of the TFT arrays can not be increased efficiently according to the conventional method of forming a polysilicon layer in a substrate. The reason is that only the polysilicon with smaller grain size instead of larger grain size can be obtained by means of the conventional method.